Electrolytic capacitors are passive elements that charge and discharge electrical charges by electrostatic capacity. Such electrolytic capacitors include a capacitor element impregnated with an electrolyte solution and placed in an outer casing, the outer casing is sealed by a seal body, and lead terminals are drawn out from the seal body. The capacitor element includes an anode foil formed of a valve metal foil such as aluminum with an oxide film as a dielectric layer formed thereof, a cathode foil formed of the same or different metal foil and facing the anode foil, and a separator between the anode foil and the cathode foil.
The anode foil is produced by performing an area increasing process on a valve metal in order to increase the surface area, and then performing a chemical conversion process of applying a voltage in a chemical conversion solution to form an oxide film on the surface of the valve metal. The cathode foil is formed by performing an area increasing process, and a chemical conversion process if necessary to form a dielectric oxide film. The separator suppresses a short-circuit of the anode foil and the cathode foil, while at the same time, holds the electrolyte solution. The separator is selected from separators mainly formed of natural fibers, such as craft paper, Manila paper, and pulp, separators mixed with synthetic fibers, and separators mainly formed of synthetic fibers.
The capacitor element is produced by connecting electrode lead terminals with the anode foil and the cathode foil respectively, and laminating and winding the anode foil and the cathode foil via the separator therebetween. The electrolytic capacitor is manufactured by impregnating the capacitor element with the electrolyte solution, inserting the capacitor element into an outer casing, sealing the outer casing, and then performing the chemical conversion again.
As for electrolytic capacitors, it is known to add a withstand voltage improving agent like polyvinyl-alcohol to the electrolyte solution (see, for example, Patent Document 1 and Patent Document 2). This is because by adding polyvinyl alcohol to the electrolyte solution, the ignition voltage of the electrolyte solution rises, and the withstand voltage of the electrolytic capacitor is improved.
When, however, polyvinyl-alcohol is added to the electrolyte solution, the viscosity of the electrolyte capacitor increases, and thus the impregnation performance of the electrolyte solution to the capacitor element decreases. That is, it becomes difficult for the electrolyte solution to be impregnated into pits formed on the anode foil and the cathode foil by the area increasing process. In this case, voids may be formed between the electrolyte solution and the internal surface of the pit regarding the pit, causing the reduction of the electrostatic capacity and the increase of a resistance of the electrolytic capacitor.
Hence, a manufacturing method has been conventionally proposed (see, for example, Patent Document 3 and Patent Document 4) of applying polyvinyl-alcohol to the separator before the impregnation of the capacitor element, impregnating the electrolyte solution into this capacitor element, and causing polyvinyl-alcohol to be dissolved in the electrolyte solution by a thermal process included in a chemical conversion process. According to this manufacturing method, since no polyvinyl-alcohol is added to the electrolyte solution at the time of impregnation, the viscosity of the electrolyte solution is not increased, facilitating the impregnation of the electrolyte solution into the pits of the capacitor element, while at the same time, improving the withstand voltage by causing polyvinyl-alcohol to be dissolved to the electrolyte solution that has been impregnated deeply inside the pits.